Automatic equalization device and method of starting-up the same

ABSTRACT

An automatic equalization device used in a data communication system includes a unit for extracting a single pulse from a training signal sent from a transmitter and initializing a tap coefficient using the extracted single pulse. The automatic equalization device also includes a first equalization circuit and a second equalization circuit. In the first equalization circuit, an auto-correlation series of the signal corresponding to the single pulse is calculated to provide a symmetric single pulse. In the second equalization circuit, an inverse matrix is calculated from the symmetric single pulse using the auto-correlational series of single pulses. By these calculations, the speed of the initial setting of the tap coefficient is increased.

BACKGROUND OF THE INVENTION

The present invention relates to an automatic equalization deviceprovided in a modem in a receiver in a digital data transmission systemand to a method of starting-up the automatic equalization device,particularly a method for starting-up an automatic equalization deviceat a high speed.

In a multipoint communication system, a master station communicatesselectively with one of a plurality of the slave stations. A transmitterand a receiver of the master station are connected through a telephonetransmission line with receivers and transmitters of the slave stations.In such a type of communication system, the transmitter of the masterstation is always connected in an on-line manner with the receivers ofthe slave stations. The master station polls each of the salve stationin sequence. Such a polled slave station transmits an "acknowledgement"signal in response to a received "request for transmission" signal andsubsequently transmits the data if the slave station has data to betransmitted.

It is necessary for a modem to adjust its receiver characteristics tothe characteristics of the telephone transmission line prior to theactual receipt of data from one of the slave stations. Thecharacteristics of the telephone transmission line vary for each slavestation, primarily because of the change of distance between the masterstation and slave stations. Once the receiver is adjusted to thetelephone transmission line, it is possible to satisfactorily track thechange of line characteristics using known, relatively slow speedmethods. However, when the receiver first receives the training data,there is a great difference in the characteristics of the receiver andthe characteristic of the line. Under such start-up conditions, it takesabout 2 seconds to adjust the receiver to the line by the known,relatively slow speed tracking method. The total adjustment time for onerequest for transmission equals the unit adjustment time multiplied bythe number of the slave stations. This total adjustment time is a keyfactor which significantly reduces the efficiency of data transmissionin prior art systems.

Various proposals have been made to provide a special start-up procedureto facilitate the quick adjustment of the receiver to the telephonetransmission line. Typical of these is the transmission from thetransmitter of a special signal series by which the receiver can derivea signal having the characteristic of the telephone transmission linefor the receiver itself. In accordance with this special signal series,a timing error signal and a phase error signal are generated. Usingthese, an equalization device or the like in the receiver is started up.

As the signal series, i.e., the training signal for the start-up of theequalization device CCITT recommendations prescribe the transmission ofa binary pseudo random data code (hereinafter referred to as a PN code).Hence, various methods have been proposed in which, to initialize thecorrection coefficient (tap coefficient) of the equalization device fromthe PN code, the correlation between the data and the error signal iscalculated and used to correct and determine the tap coefficient.However, such methods have disadvantages of requiring a large,complicated circuit for calculating the correlation and requiring aconsiderably long time for determining the tap coefficient of theequalization device. For example, about 250 msec are required from arequest for transmission to the transmission of the data.

U.S. Pat. No. 3,962,637 to David M. Motley et al. proposed a method forthe high-speed start-up of an equalization device in a receiver. In thisU.S. patent, a special signal comprising the three segments mentionedbelow is used for the training signal for the start-up of theequalization device.

That is, in the first segment, a tone signal obtained by modulating acarrier signal in a predetermined phase and amplitude is transmitted. Inthe second segment, a signal obtained by modulating the carrier signalby the data series by which data series the transmission timing, i.e.,band rate clock period, can be discriminated, is transmitted. In thethird segment a signal obtained by modulating the carrier signal bysingle pulse (impulse) data is transmitted.

On the other hand, in the receiver, automatic gain control is carriedout in the first segment of the training signal, a timing signal forsampling the data is extracted in the second segment and the singlepulse signal is extracted in the third segment.

The single pulse signal extracted in the third segment has been deformedby the transfer function (impulse response characteristic) of thetransmission line. The equalization device in the receiver compensatesthe distortion (deformation) of the signal on the line. Fundamentally,it is ideal that the equalization device have inverse characteristics ofthe transfer function of the line. Accordingly, in the above-referencedU.S. patent, the tap coefficient is calculated and determined on thebasis of the impulse response characteristics of the transmission lineextracted in the third segment of the training signal such that theequalization device has inverse characteristics of the line.

The technique disclosed in the above-referenced U.S. patent greatlyreduces the time required for initializing the tap coefficient of theequalization device in the receiver compared with the case where theabove-described PN code is used. For example, the time required from arequest for transmission to transmission of data can be reduced to lessthan 30 msec.

However, in the technique disclosed in the above-referenced U.S. patent,the impulse signal extracted in the third segment of the training signalis, in general, asymmetrical with respect to time. To obtain the tapcoefficient of an equalization device having inverse characteristicsusing such an asymmetrical signal requires more calculation time than inthe case where a symmetric signal is used.

Also, in the disclosed technique, the signal transmitted on the line inthe above-described third segment does not contain the carrier signalexcept when the single pulse signal exists. Hence, even if the phase ofcarrier signal for demodulation in the first and the second segment isadjusted, the phase is not adjusted in the third segment. Accordingly,there is the disadvantage that the phase cannot necessarily beguaranteed. Further, in the third segment, there is a possibility ofasynchronism, since no timing signal for sampling is extracted.

SUMMARY OF THE INVENTION

A first object of the present invention is to speed up the start-up ofthe automatic equalization device in a receiver and thereby improve theefficiency of data transmission in a multipoint communication system.

A second object of the present invention is to decrease the timenecessary for the initialization of the tap coefficients, in anautomatic equalization device in which the impulse responsecharacteristic of the line are used for the initialization of the tapcoefficients, by providing a first and a second equalizer unit and bymaking the impulse response characteristics symmetrical with respect totime by the first equalizer unit.

A third object of the present invention is to provide a method ofstarting up an automatic equalization device of the above-mentioned typein which the phase of the carrier signal can be guaranteed even in thesegment for extracting an impulse response signal.

A fourth object of the present invention is to provide a method ofstarting up an automatic equalization device in which the timing signalcan be extracted and hence asynchronium can be prevented even in theabove-mentioned segment for extracting an impulse signal.

In accordance with the present invention, there is provided an automaticequalization device which is arranged in a receiver for receivingthrough a transmission line and demodulating a signal obtained byquadrature-amplitude modulating a carrier signal by digital data in atransmitter, and which compensates for the distortion introduced intothe received signal by the transmission line. The automatic equalizationdevice comprises a single pulse extraction circuit for extracting asignal corresponding to a single pulse containing distortion due to thetransmission line from a predetermined training signal received forstarting up the receiver before receiving the data signal and comprisesfirst and second equalization circuits. The single pulse extractioncircuit has a means for generating a data series expressing by complexnumbers the signal corresponding to the single pulse, a means fornormalizing the data series, and a means for deriving the complexconjugates of the normalized data series. The first equalization circuithas a means for calculating an auto-correlation series of the signalcorresponding to the single pulse from the normalized data series usingthe complex conjugate data series as the tap coefficient and has a meansfor calculating a cross-correlation series between the data signalseries received after start-up and the signal corresponding to thesingle pulse using the complex conjugate data series as the tapcoefficient. The second equalization circuit has a means for obtainingthe N-th order approximate data series of the inverse matrix of theauto-correlation series using the auto-correlation series obtained inthe first equalization circuit, a means for obtaining the data seriescorresponding to the above-mentioned inverse matrix from the N-th orderapproximate data series by means of the successive approximation method,and a means for obtaining an equalized output from the cross-correlationseries of the received data signal supplied from the first equalizationcircuit by setting the data series corresponding to the inverse matrixas the initialization tap coefficient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system for data transmission and reception in whichan automatic equalization device according to the present invention isused;

FIG. 2 illustrates one example of the modulation points in thequadrature-amplitude modulation used in the system for data transmissionand reception of FIG. 1;

FIG. 3 illustrates signal waveforms of one example of the trainingsignal used in a method of starting up an automatic equalization deviceaccording to the present invention;

FIG. 4 is an abbreviated flow chart of the method of starting up anautomatic equalization device according to the present invention; and

FIG. 5 is a block diagram of the construction of an automaticequalization device in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An automatic equalization device and a method of starting up the sameaccording to the present invention will now be described with referenceto the drawings. A receiver equipped with an automatic equalizationdevice according to the present invention is illustrated in FIG. 1. InFIG. 1, when a transmitter 1 receives a transmission request signal(RS), it generates training data in a training data generating unit(TRG) 11 and supplies the training data to the modulation unit (MOD) 12.MOD 12 carries out quadrature-amplitude modulation on the carrier signalfrom a carrier signal generating unit (CRG) 13 based on the trainingdata and transmits the modulated signal to the receiver 2 through thetransmission line L. In the receiver 2, the setting of the starting-upcondition for receiving the data signal is carried out on basis of thereceived training signal. Predicting the time when the setting of thestarting-up condition will be finished, the transmitter 1 generates atransmission ready signal CS and starts to transmit the signal modulatedby the transmitting data SD to the receiver 2.

The modulation unit 12 of the transmitter 1 carries out quadratureamplitudie modulation into 16 values on the carrier signal from thecarrier generating unit 13 as shown, for example, in FIG. 2. Forreference, each signal point shown in FIG. 2 corrsponds to the amplitudeand phase of the modulated signal and can be expressed in a complexnumber notation.

The receiver 2 of FIG. 1 comprises a demodulation unit (DEM) 21 forreceiving and demodulating the signal transmitted from transmitter 1, asingle pulse extraction unit 22 for extracting a signal corresponding toa single pulse signal from the training data demodulated in thedemodulation unit 21, and a first and a second equalizer unit (EQL 1 andEQL 2) started up on the basis of the extracted data corresponding tothe single pulse signal. An example of the training data containing adata series from which a single pulse signal can be extracted is shownin FIG. 3.

In FIG. 3, TM is a timing signal for data transmission, TRD is trainingdata, and TRS is a carrier signal modulated by TRD. The training dataTRD has three segments SEG1, SEG2, and SEG3. In the first segment SEG1,the training signal TRS is a signal modulated with constant data andhence has a constant amplitude and a constant phase. Thus, in thedemodulation portion (DEM) 21 of the receiver 2, the adjustment of theautomatic gain control AGC can be carried out while this first segmentSEG1 is received. In the second segment SEG2, the training signal TRS ismodulated alternately with two data "A" and "-A" which have the oppositephase and the same amplitude. For example, such two data "A" and "-A"are shown as D_(A) and D_(-A) in FIG. 2. The timing signal for datatransmission is extracted from the signal in the second segment SEG2.

The training data in the third segment SEG3 is alternately expressed asdata "A" and "-A" except at time t₃ where the data "A" appearssuccessively. In FIG. 3, SA1 shows only a portion of the training datain the third segment SEG3, and SA2 shows that obtained by delaying it byone data symbol. By summing up the signals SA1 and SA2 in the period Twhere these two signals overlap, a single pulse series as shown in SA3,that is, a series in which not only a center element P₀ is zero, but allother elements are also zero, is obtained.

A flow chart of the start-up of the automatic equalization device of thereceiver 2 in FIG. 2 using the training signal of FIG. 3 is illustratedin FIG. 4. In the first step ○1 , the received data series (expressed incomplex numbers) corresponding to the third segment SEG3 of the trainingsignal demodulated in the demodulation unit 21 is supplied to the singlepulse extraction circuit REP. In REP, the received data series is addedto one obtained by delaying that data series by one data symbol. Thus, adata series P_(J) (J=0, ±1, . . . , ±n) as shown in SA4 of FIG. 3 isobtained. This data series P_(J) may be deemed an ideal single pulseshown in FIG. 3 distorted by the transmission line L. In the second step○2 , the data series P_(J) corresponding to the single pulse extractedas described above is supplied to the normalization circuit NR, and isnormalized in NR. The normalization circuit NR first calculates themagnitude of the data series P_(J), that is, the 0-th order correlationP², according to the following equation: ##EQU1## (herein, * denotescomplex conjugate). Then, the single pulse is normalized by dividing thedata series P_(J) by P. Assuming that the normalized data series isX_(J), then X_(J) =P_(J) /P.

In the third step ○3 , the normalized data series X_(j) is supplied tothe complex conjugate derivation circuit CN. The derived complexconjugate data series C_(J) is initialized in the first tap coefficientregister TPR1 as the tap coefficient of the first equalization circuit(EQL1) 23. Here, C_(J) =x_(J) *=P_(J) */P. In the fourth step ○4 , thenormalized data series x_(J) and its complex conjugate data series C_(J)are supplied to the computing circuit CNT1 of the first equalizationcircuit 23, and the auto-correlation series Am is calculated. Thecalculation of the auto-correlation series Am is carried out as follows.First, regarding the 0-th order correlation A₀, ##EQU2## Here, a complexnumber is expressed in the form of (real component, imaginarycomponent). Regarding the other Am's, ##EQU3## From this, it can be seenthat A_(-m) =A_(m) *. That is, the auto-correlation series A_(m) issymmetric. This auto-correlation series A_(m) can be thought of as theresult of deformation of a single pulse by the transmission line L andthe first equalization circuit EQL1. Therefore, it is required to givean inverse characteristic of the symmetric impulse characteristic in thesecond equalization circuit (EQL2) 24.

In the fifth step ○5 , the auto-correlation series A_(m) is supplied tothe computing circuit CNT2 of the second equalization circuit (EQL2) 24,and the first-order approximate series B_(J).sup.(1) of the inversecharacteristic matrix is obtained as follows.

    B.sub.J.sup.(1) =-A.sub.-J =-A.sub.J *(J≠0)

    B.sub.0.sup.(1) =A.sub.0 =(1, 0)

The series B_(J).sup.(1) obtained as above is used as the initialcondition for obtaining the inverse matrix.

In the sixth step ○6 , the data series B_(J).sup.(1) obtained in thefifth step ○5 is used as the tap coefficient B_(J) of the secondequalization circuit to calculate the equalized output S with theauto-correlation series A_(m) as the tap data. Then, the output S iscompared with the reference output series Ref, and the tap coefficientB_(J) is corrected successively so as to make the error approach zero.The equalized output S is the data series S_(L) given as follows:##EQU4## The correction of the tap coefficient B_(J) is carried out byusing the following successive approximation method: ##EQU5## Regardingother B_(J) is, ##EQU6## In addition, since the center tap is dominant,the correction of B_(J) is carried out in the following sequence:

    B.sub.0 →B.sub.±1 →B.sub.±2 →--→B.sub.±n →B.sub.0 →B.sub.±1 →--

Since the input series A_(m) of the second equalization circuit issymmetric, the data series B_(J) obtained as described above is alsosymmetric. That is, B_(J) =B_(-J) *. As described above, the tapcoefficient B_(J) of the second equalization circuit 24 is initializedand set in the tap coefficient register TPR2.

After the above-mentioned steps, the initialization of the tapcoefficient of the automatic equalization device is completed.Predicting the time when the start-up in the receiver 2 is completed asdescribed above, the transmitter 1 starts to transmit the transmissiondata. In the receiver 2, the received data signal is demodulated in thedata series in the demodulation unit 21 and supplied to the firstequalization circuit 23. In the first equalization circuit 23, the firstequalized output is calculated from the received data series by thefirst equalized output circuit OPU1 using the tap coefficient C_(J) inthe first tap coefficient register TPR1. The equalized output dataseries from the first equalization circuit 23 is supplied to theequalized output circuit OPU2 of the second equalization circuit 24, andthe final equalized output data is calculated using the second tapcoefficient B_(J). By way of reference, it is possible to form a singleequalized output circuit for the received data by calculating a combinedtap coefficient from the first tap coefficient C_(J) and the second tapcoefficient B_(J) by a convolution operation.

An automatic equalization device in accordance with one embodiment ofthe present invention is illustrated in detail in FIG. 5. In FIG. 5, thereceived signal, after demodulation by the demodulation unit 21, isequalized by the first equalization circuit 23 and the secondequalization circuit 24.

The equalized output is data discriminated in the data discriminationunit 25 and then output as output data. On the other hand, during thestart-up period, the output signal of the demodulation unit 21 issupplied to the single pulse extraction unit 22, and the first andsecond equalization circuits 23 and 24 are started up in accordance withthe extracted single pulse. The demodulation unit 21 has a filtercircuit (FIL) 211 for removing the noise from the received signal, anautomatic gain control circuit (AGC) 212 for adjusting the level of thereceived signal, a demodulation circuit (DEM) 213 for demodulation ofthe quadrature-amplitude modulation, and a roll-off filter circuit (ROF)214 for removing the high frequency component from the demodulatedsignal. The single pulse extraction unit 22 has an extraction circuit(REP) 221 for extracting the data series corresponding to the singlepulse signal from the training signal, a normalization circuit (NR) 222for normalizing the extracted data series, and a complex conjugatederivation circuit (CN) 223 for deriving the complex conjugate dataseries of the normalized data series. The first equalization circuit(EQL1) 23 has a first tap data register (TPD1) into which the receiveddata from the demodulation unit 21 is written, a first equalized outputcircuit (OPU1) 232 for calculating the first equalized output from thefirst tap data and the first tap coefficient, a first tap coefficientregister (TPR1) 234 which is initially set by the output data of thecomplex conjugate derivation circuit 223, a first computing controlcircuit (CNT1) 235 for calculating the auto-correlation series of thesingle pulse signal, and a second tap data register (TPD2) 236 intowhich the data series from the normalization circuit 222 is written. Thesecond equalization circuit (EQL2) 24 has a third tap data register(TPD3) 241 into which the first equalized output data is written, asecond equalized output circuit (OPU2) 242 for calculating the secondequalized output from the third tap data and the second tap coefficient,a second tap coefficient register (TRP2) 243, a second computing controlcircuit (CNT2) 244 for calculating the equalized output from theauto-correlation series of the single pulse and the second tapcoefficient and for correcting the second tap coefficient in accordancewith the error of the equalized output to the reference output Ref, ann-th order approximation circuit (n-th AP) 245 for obtaining the n-thorder approximation of the inverse matrix from the auto-correlationseries, a fourth tap data register (TPD4) 246 into which theauto-correlation series of the single pulse is written, and an errorcalculation circuit (EPR2) 247 for calculating the error of theequalized output to the reference output using the auto-correlationseries of the single pulse. The data discrimination unit 25 has acarrier automatic phase control circuit (CAPC) 251, a discriminationcircuit 252, and an error calculation circuit (ERR1) 253. The circuitsin the data discrimination unit 25 are disclosed, for example, inJapanese Pat. No. 1,041,066, thus are not described herein.

In accordance with the above-mentioned method of starting-up theautomatic equalization device according to the present invention, bycalculation of the symmetric inverse matrix enables reduction of thecalculation time of the tap coefficient. For example, it is possible toreduce the time required for start up compared with the prior artdisclosed in the above-mentioned U.S. Pat. No. 3,962,637.

In addition, in accordance with the method of starting-up the automaticequalization device of starting-up the automatic equalization deviceaccording to the present invention, use of a signal including thecarrier signal continuously as the signal for extracting a single pulseenables prevention of asynchronism.

INDUSTRIAL APPLICABILITY

The present invention can be applied for increasing the datatransmission efficiency in a multipoint communication system.

We claim:
 1. An automatic equalization device used in a communicationsystem in which a transmitter and a receiver are connected by atransmission line, said transmitter including a means for transmittingan initialization signal including a component of a single pulse andsubsequently transmitting a data signal, said receiver including saidautomatic equalization device for correcting the received signal andderiving the data signal transmitted from said transmitter, saidautomatic equalization device comprising:a means for extracting a singlepulse having a distortion due to the characteristics of the transmissionline from the received initialization signal; a means for deriving acoefficient for obtaining an auto-correlation or a cross-correlationseries using said extracted single pulse; a first equalization circuitfor correcting said extracted single pulse into a symmetric single pulseand correcting the data signal by the coefficient obtained by saidderiving means; a generating means for generating a coefficient forderiving a single pulse coinciding with the single pulse included in theinitialization signal transmitted from said transmitter, using saidsymmetric single pulse; and a second equalization circuit for correctingthe data signal based on the coefficient obtained by said generatingmeans.
 2. A data communication system comprising:transmitting means fortransmitting a training signal and a data signal; receiver means forextracting a single pulse signal from said training signal; autoequalization means having a coefficient set in accordance with saidsingle pulse signal, for equalizing said data signal, said autoequalization means includingmeans for obtaining a complex conjugateseries of said extracted single pulse signal; first equalization circuitmeans for calculating a series of auto-correlation of said single pulsesignal using said obtained complex conjugate series as a tapcoefficient, and for correcting said data signal; and secondequalization circuit means for calculating a data series which is theinverse matrix of said auto-correlation series, for initializing saiddata series, which is the inverse matrix as the tap coefficient, and forcorrecting said data signal using said tap coefficient.
 3. A datacommunication system according to claim 2, wherein said means forobtaining the complex conjugate series includes a normalizing means fornormalizing said single pulse signal.
 4. A data communication systemaccording to claim 2, wherein said means for obtaining the inversematrix of said auto-correlation series is derived as a first orderapproximation from the data series B_(J) using the following expressionsin the form of:

    B.sub.J =A.sub.-J  (J≠0)

    B.sub.-J =-A.sub.J  (J≠0)

    B.sub.O =A.sub.O =(1, 0)  (complex number),

where B_(J) is the J-th data of said data series and A_(J) and the J-thauto-correlation number.
 5. A data communication system according toclaim 4, wherein said means for obtaining a data series corresponding tosaid inverse matrix includes means for employing the Gaus-Seidel methodas a sequential approximation method, and said equalization circuitmeans includes means for obtaining the data series corresponding to saidinverse matrix by correcting said data series having the form of:

    B.sub.J (J=0, ±1, . . . , ±n)

in the sequence B₀, B.sub.±1, . . . , B.sub.±n and subsequently theretorepeating the correction.
 6. A method initializing a coefficient for thecorrection of a received signal in an automatic equalization device forcorrecting the received signal transmitted by a transmitter through atransmission line, said method comprising the steps of:(a) transmittingtraining signal; (b) extracting a pulse signal series corresponding to asingle pulse signal from the training signal; (c) obtaining a complexconjugate series of said pulse signal series and setting said obtainedcomplex conjugate series as a first equalization coefficient; (d)obtaining a correction series between said first equalizationcoefficient and said pulse signal series; and (e) setting the dataseries corresponding to the inverse matrix of said correlation series asa second equalization coefficient.
 7. A method according to claim 6,further comprising the step of:synthesizing said first equalizationcoefficient and said second equalization coefficient and said secondequalization coefficient into another equalization coefficient inaccordance with the convolutional operation.
 8. A method for initiallysetting a coefficient of an automatic equalization device in a datacommunication system, in which a training signal including components ofa single pulse signal is transmitted by a transmitter and a single puslesignal is extracted from said training signal by a receiver, and settingan initial coefficient of said automatic equalization device using saidextracted single pulse signal, said method comprising the steps of:(a)receiving said training signal; (b) generating a delayed trainingsignal; (c) adding the received training signal and the delayed trainingsignal to produce an extracted single pulse signal; and (d) initiallysetting a coefficient of said automatic equalization device in thereceiver using said extracted single pulse signal.